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FIRB Activities on lithium niobate : characterization of bulk materials and photoinduced effects

Fe3+ EPR spectra microRaman. FIRB Project Microdevices in Lithium Niobate –Università di Pavia. FIRB Activities on lithium niobate : characterization of bulk materials and photoinduced effects. Keypoints of our activities on LN. “crystalline quality” - characterization methods.

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FIRB Activities on lithium niobate : characterization of bulk materials and photoinduced effects

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  1. Fe3+ EPR spectra microRaman FIRB Project Microdevices in Lithium Niobate –Università di Pavia FIRB Activities on lithium niobate: characterization of bulk materials and photoinduced effects Keypoints of our activities on LN “crystalline quality” - characterization methods Some examples Microstructures in LN by fs laser irradiation Electro-optic coefficients measurements

  2. 1 2 3 FIRB Project Microdevices in Lithium Niobate –Università di Pavia Keypoints Characterization of structural, optical and electronic properties of LiNbO3 crystals and substrates in connection with different growth processes and different doping Crystalline quality Study of the transport phenomena and charge localization due to optical irradiation of LiNbO3(or other ABO3 compounds, eventually doped) and of the irradiation effects on the linear and nonlinear optical properties Study of the feasibility of 1D, 2D and 3D periodical structures, waveguides and microstructures on LiNbO3 (or other ferroelectric oxides) crystalline substrates by means of femtosecond laser irradiation in the transparent spectral region

  3. How we study crystalline quality? • Raman and micro-Raman spectroscopy • Optical absorption, PL, TL, PC, TSC • Hall, Photo-Hall and magneto-optical spectroscopy • Ellipsometry • Electron Paramagnetic Resonance (EPR)and Photo-EPR • Static magnetization measurements • Electro-optical characterization • Femto-second laser sources *

  4. strong increase of the spectrum resolution due to line narrowing • changes of some LN properties • appearance of new impurity centers EPR Raman FIRB Project Microdevices in Lithium Niobate –Università di Pavia Due to the Li-deficiency the conventional congruent crystals have high concentration of intrinsic (non-stoichiometric) defects, which can easily compensate a high concentration of extrinsic defects (for instance, optically or acoustically active impurities) coupling and mutual influence of intrinsic and extrinsic defects decrease of the intrinsic defect concentration Possibility to vary both the [Li]/[Nb] ratio and [O] contents (in addition to the modification by dopants!) is a very powerful tool for the optimisation of crystal parameters Lattice of ideal, defect-free LN crystal

  5. FIRB Project Microdevices in Lithium Niobate –Università di Pavia • EPR spectroscopy : • Control of the material quality: • check of purity of growth processes • detection of defects and/or unwanted EPR active magnetic impurities • information about structural disorder • Evaluation of the oxidation state of the transition ions • Information about site symmetry from the EPR signal angular dependence Fe3+ EPR lines (B//c) in CLN (LN:Fe 0.1%) …in quasi-st LN (LN:Fe 0.1%)

  6. In crystals, Raman spectrum depends on the direction and polarization state of the incident and scattered light with respect to the cristallographic axes FIRB Project Microdevices in Lithium Niobate –Università di Pavia Raman in LiNbO3 • Porto notation: ki(ei,ed)kd The crystal structure of pure LiNbO3 has Rc3 space group symmetry and 4A1+ 9E Raman-active modes are predicted by factor-group analysis

  7. FIRB Project Microdevices in Lithium Niobate –Università di Pavia RS is strongly sensitive to orientation Elight| c Elight // c m-Raman to check disorientation, multidomains…

  8. FIRB Project Microdevices in Lithium Niobate –Università di Pavia RS is sensitive to the deformation of the lattice and to the presence of point defects, becoming a powerful tool to deal with the problem of stoichiometry The mode at 880 cm-1 is the vibration, parallel to the c axis, of the oxygen ions which consists basically in the stretching of the Nb–O and Li–O bonds. When a Nb ion sits at a Li site its oxygen first neighbors increase their bonding forces respective to the perfect crystal situation because of the stronger electrostatic interaction.

  9. The fact that the linewidth of some Raman modes scale with the composition xc = [Li/([Li] + [Nb])of LN crystals, together with the use of a confocal microscope (microRaman spectroscopy), allow a three dimensional estimation of the sample stoichiometry. FIRB Project Microdevices in Lithium Niobate –Università di Pavia RS can be used to check the stoichiometry (Li/Nb ratio) monitoring the changes of linewidth of some Raman modes.

  10. mRaman for surface quality analysis after wafering process: • Non-destructive stuctural tool • Micron-scale spatial resolution • Presence of a structurally disordered layer • Effectiveness of damage removal method • Control on optical surface finishing FIRB Project Microdevices in Lithium Niobate –Università di Pavia

  11. Important complete characterization of: stoichiometry, nature and content of impurities, degree of structural disorder before starting with investigation of charge trapping mechanisms and phenomena related to photo-induced defects 2 FIRB Project Microdevices in Lithium Niobate –Università di Pavia • Photovoltaic current, photoconductivity, • Photo-EPR • vs %, doping, l, T Study of the transport phenomena and charge localization due to optical irradiation of LiNbO3(or other ABO3 compounds, eventually doped) and of the irradiation effects on the linear and nonlinear optical properties

  12. “MICROSTRUCTURAL MODIFICATION OF LINBO3 CRYSTALS INDUCED BY FEMTOSECOND LASER IRRADIATION” Appl. Surf. Science in press 3 FIRB Project Microdevices in Lithium Niobate –Università di Pavia Study of the feasibility of 1D, 2D and 3D periodical structures, waveguides and microstructures on LiNbO3 (or other ferroelectric oxides) crystalline substrates by means of femtosecond laser irradiation in the transparent spectral region

  13. Laser system 1 (high energy, low repetition rate): Amplified Ti:Sapphire (1 mJ-130 fs-1 kHz) FIRB Project Microdevices in Lithium Niobate –Università di Pavia Activity of Pavia Unit in fs-laser writing femtosecond irradiation ofcongruent LN as a function of pulse energy, exposure time, exposure depth, crystal orientation, etc. . characterisation via in situ optical microscopy and a posteriori micro-Raman spectroscopy Laser system 2 (low energy, high repetition rate): Ti:Sapphire oscillator (25 nJ-130 fs-82 MHz) energy deposition through multi-photon absorption  energy transfer strongly depends on pulse intensity

  14. FIRB Project Microdevices in Lithium Niobate –Università di Pavia Microstructures in LiNbO3 crystals by fs laser irradiation (laser 1) • 2-mm-diameter holes in a z-cut CLN plate (laser 1, 10 ms, ~1mJ, LWD 50X microscope objective lens) b) c) a) • 50-mm-diameter hole in a z-cut CLN plate (laser 1, 10 s,50 mJ, 63x microscope objective lens) • same as a) imaged by a polarizing microscope

  15. FIRB Project Microdevices in Lithium Niobate –Università di Pavia Microstructures in LiNbO3 crystals by fs laser irradiation (laser 2) • 125-mm-diameter hole in a z-cut CLN plate (laser 2,30s, 10 nJ, 63x microscope objective lens) • same as a) imaged by a polarizing microscope. A bright zone aside the hole is visible due to photo-induced birefringence.

  16. 1 2 3 4 FIRB Project Microdevices in Lithium Niobate –Università di Pavia Micro-Raman investigation of microstructures formed by laser 2 4 635 cm-1 3 880 cm-1 2 20 mm 1 2D mapping of Raman intensity in the square The brighter the colours the larger the Raman line intensity Image of a hole in z-cut CLN plate (laser 2, 0.01s, 5 nJ, 20x objective lens). The maximum depth of the hole is 10 mm z(xx)z Raman spectra recorded at positions 1 to 4 as shown in the top left side image. A1 symmetry-forbidden Raman lines appear as approaching the edges of the microstructure indicating some orientation changes in the crystal structure

  17. 3 2 1 10 mm FIRB Project Microdevices in Lithium Niobate –Università di Pavia Micro-Raman investigation of microstructures formed by laser 1 Image of a microstructure in a z-cut CLN plate (laser 1, 10s, 300mJ, 63x microscope objective lens) Raman spectra recorded in zone 1 to 3. The main E-type peaks are strongly quenched while the A1 peak at 635 cm-1 increases. In spectrum 3 even Nb-O related vibrations at frequency larger than 500 cm-1 are absent, as it would happen in an amorphous layer

  18. FIRB Project Microdevices in Lithium Niobate –Università di Pavia Conclusion femtosecond irradiation induces disorder in the crystal structure causing the appearance of Raman peaks of forbidden symmetry niobium oxides are formed in the ablation process with laser system 2 amorphous surfaces are present in the region ablated by means of laser system 1 Ablation edges of microstructures formed by laser system 2are smooth and a strong induced birefringece is present all around. High-intensity ultra-short pulses from laser system 1probably leads to the formation of an electron plasma and localized optical breakdown, whereas charge accumulation and photorefractive-like damage may be the mechanism excited in the case of the high-repetition-rate-low-energy fs pulses from laser system 2. In both cases multi-photon absorption is the path for energy transfer into the medium

  19. FIRB Project Microdevices in Lithium Niobate –Università di Pavia EO coefficients of Lithium Niobate Class 3m r33, r13= r23, r22= -r12=-r61, r42=r51, rc=r33-(no/ne)3r13 i =1,6 j =1,3 Measuring techniques may rely on ellipsometry or interferometry with DC or AC applied electric field Constant stress (rT) or constant strain (rS) EO coefficients are measured when the electric field frequency is below/above the acoustic resonance of the crystal (above 500KHz) r in pm/V at l=633nm

  20. FIRB Project Microdevices in Lithium Niobate –Università di Pavia EO coefficients of Lithium Niobate r33 rc r13 EO coefficients of Crystal Technology CLN (empty symbols) and SLN (full symbols) with xc=0.497 provided by the Crystal Growth laboratory- Universidad Autonoma de Madrid

  21. FIRB Project Microdevices in Lithium Niobate –Università di Pavia EO coefficients of commercial SLN SLN wafer from OXIDE Co. Japan with nominal xc=0.50 (kindly provided by project partner AVANEX) r in pm/V ± 5%, l=633nm from AC field ellipsometry from AC field interferometry OXIDE data sheet: r33=38.3 r13= 10.4

  22. FIRB Project Microdevices in Lithium Niobate –Università di Pavia Contradictory results from the literature K. Chah et alii, APB 67 (1998) 65 Y. Kondo et alii, J.JAP 39 (2000) 1477 Different trends were measured for LN EO coefficient as a function of 100.xc

  23. Ec hnIR Ev FIRB Project Microdevices in Lithium Niobate –Università di Pavia Femto-second laser writing and sculpturing The extremely high power density (> TW/cm2) of focussed fs pulses easily excites multi-photon absorption avalanche ionization optical breakdown leading to ablation or refractive index changes in transparent media Long penetration depth and low thermal damage open the way to microstructuring in bulk materials Perspective of micro-channels and holes, 3D gratings, buried waveguides, 3D directional couplers etc A lot of work in glass but still few examples in LN

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